1. Technical Field
This invention relates to industrial process control instrumentation, more particularly, for a method and apparatus for electrically isolating interrelated vortex electronic circuitry.
2. Background Art
It has been known for many years that vortices are developed in a fluid flowing past a non-streamlined obstruction. It also has been known that with certain arrangements vortices are developed by alternately shedding at regular intervals from opposite edges of the obstruction to form corresponding rows of vortices. Such vortices establish a so-called von Karman "vortex street," which is a stable vortex formation consisting of two nearly-parallel rows of evenly-spaced vortices travelling with the flow stream.
In a von Karman vortex street, the vortices of one row are staggered relative to those of the other row by approximately one-half the distance between consecutive vortices in the same row. The spacing between successive vortices in each row is very nearly constant over a range of flow rates, so that the frequency of vortex formation is correspondingly proportional to the velocity of the fluid. Thus, by sensing the frequency of vortex formation it is possible to measure the fluid flow rate. Devices for that purpose are often referred to as vortex meters or vortex flowmeters.
Various types of vortex meters have been available commercially for a number of years. Typically, these vortex meters comprise a vortex-shedding body mounted in a flow tube together with a sensor for detecting the frequency of vortex formation. Sensors used to detect the vortices often include diaphragms which fluctuate in response to alternating differential pressure variations generated by the vortices. The pressure applied to the diaphragms is transferred to a sensor or transducer which then produces electronic signals responsive to differential pressure applied to the diaphragms. This differential pressure measurement is used, in turn, to measure the frequency of vortex formation and ultimately the fluid flow rate or velocity.
Typically, the sensor produces an AC sinusoidal voltage signal which is linearly proportional to the volumetric flow rate. This signal is conditioned and amplified for transmission by electronic circuitry located in a housing mounted integral with the flowmeter body. The electrical components in the housing transforms the sinusoidal signal to a square wave pulse train of constant voltage amplitude and having a frequency equal to the vortex shedding frequency. This signal can then be transformed to a 4 to 20 mA dc signal that is directly proportional to the frequency of the square-wave signal, and in turn, directly proportional to the frequencies of the vortices sensed by the sensor. The final output signal can be available in either pulse form with each pulse representing a discrete quantity of fluid from which the volumetric total can be derived or, optionally, as a 4 to 20 mA dc signal for flow rate recording or control.
Often, the vortex sensing element produces signals which are referenced to a local earth ground, that being, a signal having a ground common to the meter body and the process piping. In these cases, an electrical isolation barrier is designed into the vortex meter electronics to electrically isolate the circuitry sharing the sensor ground from the electronics sharing the transmitter power supply ground. This is necessary to break ground loops which can exist when the transmitter is located a considerable distance from the power supply.
The nature of the vortex flowmeter imposes additional design constraints on the electrical isolation barrier. First, the circuitry requires low power consumption in order to adhere to industrial instrumentation standards for generating the 4-20 mA signal. Preferably, micro-power consumption in the range of 0-10 mW is desired. Second, in hostile environments, intrinsic safety standards need to be met. Examples of such standards include, but are not limited to, the Factory Mutual System Standard 3610 for Hazardous Locations and the CENELEC Intrinsic Safety Standard For Electrical Apparatus For Potentially Explosive Atmospheres, EN50020. These standards impose certain physical as well as electrical isolation requirements which ensure that even under fault conditions, the electronics are incapable of causing a spark or thermal effect which could ignite a flammable mixture or combustible material. Furthermore, to reduce the cost of the flowmeter, it is desirable to utilize low cost electrical components and to reduce the complexity of the electrical circuitry. It is also advantageous to reduce the complexity of the electrical circuitry since industrial instrumentation standards limit the amount of space that can be used for packaging the electronics. Therefore, it is desirable for an isolation barrier designed for use in a vortex flowmeter to consume low power, adhere to intrinsic safety standards, utilize less circuit board area, be comprised of low cost components, and to contain a minimum number of electrical isolation barrier crossings.
Various analog isolation devices are well known in the art utilizing electromagnetic coils, capacitive isolation techniques, and optical couplers. Monolithic integrated circuits utilizing these techniques consume large amounts of power, typically greater than 10 mW, which makes them less suitable for industrial instrumentation. Another type of analog isolation technique is one which performs the signal conditioning of the raw sensor signal, and then transforms the signal to a square-wave pulse signal which is then passed across an isolation barrier. This technique has several disadvantages. First, isolation barriers used in this technique are usually positioned at a later stage in the processing. This may increase the number of barrier crossings when more efficient signal conditioning techniques are employed. Further, the waveform's spectral information is lost thereby limiting the use of the signal in further signal processing stages. Accordingly, there has existed a need for an improved isolation barrier which not only isolates the different electrical subcomponents used in generating the final output signal from a vortex flowmeter, but also adheres to the constraints of low power utilization, low cost component construction, intrinsic safety requirements, and which minimizes the number of barrier crossings.
It is an object of the present invention to provide a circuit isolation technique for isolating a grounded low frequency AC sinusoidal signal from electronic circuitry having a different ground.
A further object of the present invention is to provide a circuit isolation technique for converting an unconditioned AC sinusoidal signal at an initial ground potential to a representative AC sinusoidal signal at a second ground potential.
Another object is to provide a circuit isolation technique of the type described which is of low cost component construction and which requires minimal power consumption.
Another object is to provide a circuit isolation technique of the type described which minimizes the number of barrier crossings required to carry over the initial vortex signal.
Another object is to provide an infallible circuit isolation technique of the type described which adheres industrial intrinsic safety requirements.
Other general and specific objects and advantages of this invention will be apparent and evident from the accompanying drawings and the following description.